1. Field of the Invention
The present invention relates to thin-film coated optics, particularly to reflective multilayer-coated optics, and more particularly to a method of producing low-cost EUV optics.
2. Description of Related Art
Many applications, including extreme-ultraviolet (EUV) lithography, astronomy, microscopy, spectroscopy and related imaging techniques require thin-film coatings deposited with a highly controlled thickness distribution over curved optical surfaces of various sizes, shapes and figures. The thickness distribution required for a given optic can be obtained with several deposition techniques. For example, one can use a precisely shaped shadow mask in front of the optical surface or baffles may be place over the deposition sources to tailor the deposition flux distribution. One can also use velocity modulation techniques in which the optical surface to be coated is swept in front of the deposition source in a controlled manner. Numerous other methods using different approaches to get the required thickness distribution exist.
Optics meeting the aspheric figure and finish specifications of today""s advanced optical systems can be extremely expensive. The cost of the condenser optics in an EUV stepper has been very high., especially since it is the lifetime of these mirrors is limited by the radiation and debris from the laser plasma source. The cost of the mirrors obtained up to now is around $500,000 with barely acceptable roughness. Diamond-turned mirror substrates made of Al can be obtained at a small fraction of this cost with a figure well within the specification; however the finish is not sufficiently smooth to allow deposition of multilayer coatings which have high reflectance at EUV wavelengths.
It is an object of the present invention to provide a method for making a mirror substrate using an inexpensive fabrication technique and then depositing a smoothening overcoat layer on the substrate to obtain a required surface smoothness.
It is another object to form a condenser mirror for use in an EUV lithography process, wherein the mirror is formed on an inexpensively fabricated substrate and then depositing a smoothening overcoat layer on the substrate to obtain a required surface smoothness.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
Spheric and aspheric precision mirrors with a surface finish in the Angstrom range are very expensive when produced by optical polishing and figuring. For example a set of mirrors for the C1 condenser of the EUV stepper of the EUV LLC costs close to $500,000. Diamond turning of metals, on the other hand, can produce aspheric mirrors with good figure at a much lower price (around $1000), but with a finish (roughness around 0.1 micron) that does not permit deposition of high reflectance multilayer coatings. In the present invention, applying a coating starting from a liquid by spin- or dip-coating can smooth diamond turned aluminum mirrors. This film and the reflecting multilayer on top of it can later be removed chemically to make the substrates available for a fresh coating. Due to the low price of diamond turned optics one can also consider replacing the entire condenser optics with a new one on a regular schedule. This can reduce the cost of ownership of EUV lithography tools
A version of this invention was used to produce low-cost surrogates required for the development of the deposition processes of high-precision thin film coatings. It has significantly lowered the cost of developing the multilayer coating processes of various extreme-ultraviolet lithography optics. The present invention is useful for fabricating condenser mirrors for the ETS EUV (Engineering Test Stand extreme-ultraviolet) stepper. EUV lithography is expected to have a significant impact on the further miniaturization of electronic devices, which would have defense applications in addition to business and domestic applications. Also, this invention is useful in applications that require optical systems with thin-film-coated optics, like those used in the National Ignition Facility (NIF).
Several fields of technological and scientific importance require optical systems with reflective optics. This method can be used to develop precision multilayer coatings for EUV lithography and precision graded optical coatings for general applications. In addition, these mirrors are used for x-ray laser cavities and optics, optics for light sources including laser-produced plasmas and synchrotrons, and also as optics for x-ray microscopy and astronomy.
The use of spin- or dip coated films to smooth the surface of an optic to sub-nanometer RMS level is probably new.
The use of aluminum mirrors fabricated to the right spheric or aspheric shape with a diamond turned technique and smoothed out with a spin- or dip coated film layer to improve the surface finish and make it acceptable for EUV reflectors is definitely new
The present invention provides a solution to the above-mentioned problem of high cost mirrors for the process development of precision thin film coatings. This invention involves a method that provides a re-usable mirror made of aluminum with a relatively inexpensive fabrication technique. The method consists of fabricating the aluminum mirror with a metal machining technique to get the desired substrate figure and to coat the mirror with a smoothening layer of material to obtain the desired surface smoothness. The key of this invention is to deposit a thin layer that will improve the smoothness of the surface to sub-nanometer root-mean-square (RMS) roughness levels. The invention has been experimentally verified with diamond-turned aluminum mirrors that were coated with a layer of standard microelectronic photoresist. It has been shown that these photoresist-coated diamond turned aluminum mirrors can be used, and re-used, in the several iterative steps of the development of the deposition processes for high-reflectance multilayer coatings for EUV lithography optics. Photoresist has been chosen for this application because of the ease of using acetone to remove the film and the coating placed on the film.
Polyimide has been tested as a smoothing layer for the possible application in cheap, possibly throwaway replacement mirrors for illumination (condenser) systems in EUV steppers. Polyimide has considerably higher radiation, thermal and chemical stability and compatibility with a high vacuum environment as compared to photoresist. Another attractive candidate is spin-on glasses, but those have not been tested.